The Geochemistry Of Magmatic Rocks In Ningzhen Area

Ningzhen metallogenic distrcit is located at the easternmost end of the lower Yangtze River Belt?LYRB?,and is one of the seven major mineralization districts in the LYRB.The magmatic activity in the Ning zhen area is frequent,and the Mesozoic magmatic rocks are widely developed.But related researches of this area are in the preliminary stage.In this study,systematic studies on zircon chronology and petrogeochemistry were mainly conducted in Yeshan area and the eastern part of Ningzhen area.The main rock types in Yeshan area are gabbro,gabbro diorite and monzogranite.Field geology shows that there are two kinds of rocks exposed in the north and south of Yeshan.The zircon ²⁰⁶Pb/²³⁸U weighted average ages of Yeshan?YS? and Yeshan?tailings reservoir:YSW? are 127.6-124.6Ma and 127.6+/-1.0Ma,respectively,which were formed in the early Cretaceous.In terms of geochemistry,the intermediate-acid magmatic rocks in the south and north of Yeshan belong to the adakite rocks with high magnesium content,which may be formed in the process of the partial melting rise of the lower crust of the deformed crust reacting with the mantle peridotite.The range of values of zircon Hf?t?is-19--5).Ye shan tailings reservoir-20--16.The T_DM2 values are 1.1-2.0Ga and 1.8-2.1Ga,respectively.The rocks in the north of Yeshan have higher oxygen fugacity and lower zircon Ti temperature than those in the south of Yeshan,indicating that the former was formed in the source area with relatively fluid rich and high oxygen fugacity,and the Yeshan iron-boron deposit may be related to the rocks in the north of Yeshan.The contrating geochemical characteristics between the north and the south of Yeshan may indicate the existence of"contemporaneous and different sources"magmatic rocks in Yeshan area.These two different rocks may be formed in the suture zone of the collision of the North and South China continental blocks.Ningzhen metallogenic distrcit is mainly divided into eastern and western parts,of which the western part is mainly exposed medium-acidic adakite,which is related to copper-molybdenum iron ores,which has been well studied in recent years,while the eastern part of Ningzhen is relatively no well studied.This paper selects the exposed magmatic rocks of Yushan and Longwangshan and studies the rock formation age,source area and tectonic setting.The rocks in the eastern part of Ningzhen are also high-Mg adakitic rocks.The weighted averaged age of ²⁰⁶Pb/²³⁸U of zircon is107.8+1.2Ma?n=8?in Yushan,105.2+1.5Ma?n=13?in Longwangshan,which belongs to the late Early Cretaceous and is obviously later than the intrusive rocks in Yeshan area.The zircon ?Hf?t? values of the two rock masses range from-24.8 to-13.2,indicating that the magma has more ancient lower crust material involved.Averaged Ti in zircon temperature is 689?.The ziron Ce⁴⁺/Ce³⁺ratio ranges from 3.33 to 400,with an average value of ?93,which is generally characterized by low oxygen fugacity.The characteristics of magmatic rocks in the eastern part of Ningzhen are similar to those of ore-barren adakitic rocks in the South Tan-Lu Fault.They are characterized by low Ce/Pb and Sr/La ratios,low Ce,Eu anomalies and low?Hf?t?,which are the characteristics of the ancient lower crust.Their higher Mg# may indicate a delamination of the thickened continental crust under a subduction setting.Combined with previous published chronological and geochemical characteristics of the Mesozoic magmatic rocks in the Central and Eastern China,the multi-period high-magnesium adakites exposed in Ningzhen area and their significant geochemical characteristics may be derived from the lower crust of South China block and North China block or the source area of lithospheric mantle metasomatized by subduction.The subduction of the Pacific Ocean at 130Ma-125Ma may from the delaminatated older crust which was formed by early collision between South China plate and the North China plate in Yeshan area.The adakitic rocks in the eastern part of Ningzhen may form by the delamination of the thickened continental crust due to the high-angle subduction of the Pacific plate.The central and eastern part of China has been under the background of the Paleo-Pacific Subduction in the Yanshanian period.